Stability of inviscid shear shallow water flows with free surface is studied in the framework of the Benney equations. This is done by investigating the generalized hyperbolicity of the ...integrodifferential Benney system of equations. It is shown that all shear flows having monotonie convex velocity profiles are stable. The hydrodynamic approximations of the model corresponding to the classes of flows with piecewise linear continuous and discontinuous velocity profiles are derived and studied. It is shown that these approximations possess Hamiltonian structure and a complete system of Riemann invariants, which are found in an explicit form. Sufficient conditions for hyperbolicity of the governing equations for such multilayer flows are formulated. The generalization of the above results to the case of stratified fluid is less obvious, however, it is established that vorticity has a stabilizing effect.
Full text
Available for:
BFBNIB, INZLJ, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK, ZRSKP
Experimental syntheses of two iron polyhydrides FeHx(I) and FeHx(II) have been carried out in diamond anvil cells by laser heating of metallic iron to temperatures of about 700 and 2000 K at ...pressures of 178 and 195 GPa, respectively. The initial sample is an iron plate enriched in the Fe-57 Mössbauer isotope placed in ammonia borane (BH3NH3). The electronic properties of FeHx compounds have been studied by measuring the electrical resistance R(T) at high pressures (180–216 GPa) in the temperature range of ~8–300 K. Based on the obtained R(T) data, two superconducting phases of FeHx compounds with the maximum critical transition temperatures Tc ≈ 25.0 and 27.7 K have been identified. It has been found that with increasing pressure, the temperature Tc in both hydrides increases linearly with the coefficients dTc/dP ~ 0.063 ± 0.001 K/GPa and 0.056 ± 0.003 K/GPa for the FeHx(I) and FeHx(II) phases, respectively. Superconductivity in iron hydrides revealed by the measured resistance R(T) has been confirmed by a number of additional methods.
We derive a mathematical model of shear flows of shallow water down an inclined plane. The non-dissipative part of the model is obtained by averaging the incompressible Euler equations over the fluid ...depth. The averaged equations are simplified in the case of weakly sheared flows. They are reminiscent of the compressible non-isentropic Euler equations where the flow enstrophy plays the role of entropy. Two types of enstrophies are distinguished: a small-scale enstrophy generated near the wall, and a large-scale enstrophy corresponding to the flow in the roller region near the free surface. The dissipation is then added in accordance with basic physical principles. The model is hyperbolic, the corresponding ‘sound velocity’ depends on the flow enstrophies. Periodic stationary solutions to this model describing roll waves were obtained. The solutions are in good agreement with the experimental profiles of roll waves measured in Brock’s experiments. In particular, the height of the vertical front of the waves, the shock thickness and the wave amplitude are well captured by the model.
Actor-Transformers for Group Activity Recognition Gavrilyuk, Kirill; Sanford, Ryan; Javan, Mehrsan ...
2020 IEEE/CVF Conference on Computer Vision and Pattern Recognition (CVPR),
06/2020
Conference Proceeding
Open access
This paper strives to recognize individual actions and group activities from videos. While existing solutions for this challenging problem explicitly model spatial and temporal relationships based on ...location of individual actors, we propose an actor-transformer model able to learn and selectively extract information relevant for group activity recognition. We feed the transformer with rich actor-specific static and dynamic representations expressed by features from a 2D pose network and 3D CNN, respectively. We empirically study different ways to combine these representations and show their complementary benefits. Experiments show what is important to transform and how it should be transformed. What is more, actor-transformers achieve state-of-the-art results on two publicly available benchmarks for group activity recognition, outperforming the previous best published results by a considerable margin.
•To exploit both the spatial and temporal correlations in a video, we hardwire convolutions in the soft-Attention LSTM architecture.•We introduce motion-based attention which guides better the ...attention towards the relevant spatial-temporal locations of the actions.•We demonstrate how the attention generated from our VideoLSTM can be used for action localization by relying on the action class label only.•We show the theoretical as well as practical merits of our VideoLSTM against other LSTM architectures for action classification and localization.
We present VideoLSTM for end-to-end sequence learning of actions in video. Rather than adapting the video to the peculiarities of established recurrent or convolutional architectures, we adapt the architecture to fit the requirements of the video medium. Starting from the soft-Attention LSTM, VideoLSTM makes three novel contributions. First, video has a spatial layout. To exploit the spatial correlation we hardwire convolutions in the soft-Attention LSTM architecture. Second, motion not only informs us about the action content, but also guides better the attention towards the relevant spatio-temporal locations. We introduce motion-based attention. And finally, we demonstrate how the attention from VideoLSTM can be exploited for action localization by relying on the action class label and temporal attention smoothing. Experiments on UCF101, HMDB51 and THUMOS13 reveal the benefit of the video-specific adaptations of VideoLSTM in isolation as well as when integrated in a combined architecture. It compares favorably against other LSTM architectures for action classification and especially action localization.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, UL, UPCLJ, UPUK
We derive a dispersive model of shear shallow water flows which takes into account a non-uniform horizontal velocity. This model generalizes the Green–Naghdi model to the case of shear flows. Besides ...the classical dispersion term in the Green–Naghdi model related to the acceleration of the free surface, it also contains a new dispersion parameter related to the flow structure. This parameter is related to the second moment of the velocity fluctuation with respect to the vertical coordinate. The distinction between shearing and turbulence based on the scale of variation of the velocity fluctuation is proposed. In particular, an equation for the turbulence generation is derived. Solitary waves for this model are obtained in explicit form. Comparison of solitary wave profiles with experimental ones is also performed. The agreement is very good apart from the small region near the top of the wave.
A conservative hyperbolic two-parameter model of shear shallow-water flows is used to study the classical turbulent hydraulic jump. The parameters of the model, which are the wall enstrophy and the ...roller dissipation coefficient, are determined from measurements of the roller length and the deviation from the Bélanger equation of the sequent depth ratio (experimental data by Hager & Bremen, J. Hydraul. Res., vol. 27, 1989, pp. 565–585; and Hager, Bremen & Kawagoshi, J. Hydraul. Res., vol. 28, 1990, pp. 591–608). Stationary solutions to the model describe with a good accuracy the free-surface profile of the hydraulic jump. The model is also capable of predicting the oscillations of the jump toe. We show that if the upstream Froude number is larger than
${\sim }1. 5$
, the jump toe oscillates with a particular frequency, while for the Froude number smaller than 1.5 the solution becomes stationary. In particular, we show that for a given flow discharge, the oscillation frequency is a decreasing function of the Froude number.
•Wave coarsening for roll waves down an inclined plane is demonstrated.•Morphological instability of long wave trains on shear ows is observed.•Stability of a single 1-D wave under multi-D ...perturbations is established.
The formation of a periodic roll-wave train in a long channel is studied for two sets of experimental parameters corresponding to Brock’s experiments 3, 4 who measured permanent wave profiles by introducing periodic perturbations at the channel inlet. In both cases, a formed free surface profile is found in good agreement with the experimental results. Qualitative properties of solutions to the model are studied in the case where the perturbation frequency is lower than the experimental one, so longer waves are generated at the channel inlet. It is observed that the corresponding roll-wave train is strongly modulated. In the case where the waves of two different lengths are generated at the channel inlet, the coarsening is observed, i.e. the process where shorter waves disappear progressively by transferring their energy to longer waves forming later a regular roll-wave train. The coarsening phenomenon is always accompanied by a strong modulation. A comparison with the Saint-Venant equations is also performed. The formation of a single wave composing a roll-wave train is also studied in a domain with periodic boundary conditions (called “periodic box”) for the same sets of experimental parameters. The free surface profile is found also in good agreement with the experimental results. This allows us to justify the use of the “periodic box” as a simple mathematical tool for a qualitative study of stability of roll-waves. In particular, we study the stability of a single steady wave by increasing its length. It is shown that the wave becomes morphologically unstable after some critical wave length: it bifurcates into a system of two waves. In the framework of a simplified multi-D model of shear shallow water flows it is also proved that a single steady wave is stable under multi-dimensional perturbations.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•A positive field sensitivity of ΔE-effect in the FCCSB ribbons is observed.•The acid etching induces a partial surface crystallization.•The acid etching enhanced thermal stability in ΔE-effect and ...field sensitivity.•The acid etching of 40 min before a 390 °C annealing give better results.
We study the thermal stability of ΔE-effect and field sensitivity on the Fe67Co10Cr3Si5B15 (FCCSB) ribbons with different acid-etching time. The testing temperature (T) range is from 30 to 240 °C. For all samples, we observed a general feature of ΔE-effect increasing with magnetic field up to a saturation value. The slope of ΔE versus field, defined as field sensitivity, is positive in contrast to that of the samples without Cr, suggesting the Cr-adding can alter the lattice response to the field from shrink to expansion due to the antiferromagnetic interaction of Cr-Fe. Without acid treatment, the sample annealed at 390 °C under field of 2000 Oe yields a maximum ΔE-effect. With the additional acid treatment for 40 mins before annealing, the thermal stabilities of ΔE-effect and field sensitivity are enhanced. Cross section TEM images provide an evidence for the etching induced surface crystallization. Thus, we attribute the etching enhanced thermal stability of ΔE properties to the modification of interfacial stress. This study provides useful information for the application of FCCSB ribbons as a field sensor with anti-corrosion properties.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
PDF
